1. Field of the Invention
The invention relates to a method and circuit for correcting defect pixels in an image signal.
2. Description of the Prior Art
Recently, with the image sensor resolution increasing to the mega-pixel stage, defect pixels (DP) has become inevitable. And, because of defect pixels, the yield rate of manufacturing mega-pixel image sensors is extremely low. However, if the problem can be recovered by some post image processing technique, the yield rate will be improved apparently.
In prior art, two major methods are adopted for correcting defect pixels. The first method is Static Defect Pixel Correction (SDPC). This method is necessary to calibrate sensors for getting position information about defect pixels before entering the market and record the position information through a suitable non-volatile storage device. Since the sensors have been calibrated and the defect pixel information has been recorded, each of the defect pixels will be corrected during defect pixel correction process.
The second method is Dynamic Defect Pixel Correction (DDPC). This method is unnecessary to calibrate sensors in advance and the non-volatile storage device is not required to record the information about defect pixels. After outputting a pixel of an image frame, the method consults neighbor pixels around the pixel in detecting to judge whether the pixel is a defect pixel or not. Due to different image contents, the non-defect pixel is possible to be misjudged as a defect pixel. In general, the accuracy of the dynamic defect pixel correction is lower than that of the static defect pixel correction.
Most algorithms, such as U.S. Pat. No. 6,965,395 and U.S. Pat. No. 7,199,824, focus on how to find defect pixels accurately and how to compensate the defect pixels correctly with DDPC. However, continuous defect pixels may be a problem when utilizing DDPC. Because of continuous defect pixels, DDPC will be unable to compensate the defect pixels effectively. Because not all neighbor pixels used for compensating the defect pixel are non-defect pixels, the pixels used for correcting may be a defect pixel before the neighbor pixels are judged whether they are defect pixels or not. Therefore, the continuous defect pixels are unable to be corrected and compensated through DDPC.
Please refer to FIG. 1A through FIG. 1C. FIG. 1A through FIG. 1C are schematic diagrams illustrating DDPC of the prior art. As shown in FIG. 1A through FIG. 1C, pixel 10 is being detected and judged as a defect pixel. In order to correct the pixel 10, DDPC utilizes neighbor pixels 11 through 18 to compensate the pixel 10. The neighbor pixel 12 is a detected pixel and a defect pixel, so the pixel 12 will be excluded to compensate the pixel 10. However, the neighbor pixels 15 through 18 are not detected yet, and the neighbor pixels 15 and 17 are defect pixels. Therefore, while compensating the pixel 10, the defect pixels 15 and 17 will be used for compensation on account of unknown. Accordingly, the pixel 10 still can not be corrected accurately after DDPC.
Furthermore, SDPC of the prior art utilizes a recorded list about defect pixel to compensate the defect pixel by simply using an averaged luminance of all neighbor pixels around the defect pixel. Although positions about the defect pixels and the non-defect pixels are known and the defect pixels can be corrected consequently to avoid utilizing defect pixel to correct the defect pixel, the defect pixel still can not be compensated effectively.
Accordingly, the main scope of the invention is to provide a method and circuit for correcting defect pixels in an image signal, so as to solve the aforesaid problems.